The invention relates to a photo cathode for use in a vacuum tube at least comprising a cathode layer, having an entrance face capable for absorbing photons impinging on said cathode layer, and an exit face for releasing electrons upon impinging of said photons, as well as an electron exit layer, in facing relationship with said exit face of said photo cathode layer for improving said releasing of said electrons.
The invention also relates to a vacuum tube with a photo cathode according to the invention.
Please note that in this application vacuum tube structures comprise—amongst others—sealed devices like image intensifiers and photo multipliers that incorporate elements or subassemblies like discrete dynodes and microchannel plates that use the phenomenon of secondary emission as a gain mechanism.
Such vacuum tubes are known in the art. They comprise a cathode which under the influence of incident radiation, such as light or X-rays or other elementary particles (electrons), emits electrons, like for example photo electrons, which move under the influence of an electric field towards an anode. The electrons striking the anode constitute an information signal, which signal is further processed by suitable processing means.
Electron Affinity (EA) is a physical parameter and proposes the energy a free electron will loose when it is emitted from the cathode to the vacuum. The value of the electron affinity is determined, among others, by the material properties of the cathode. Most materials have a positive electron affinity and yield a very low quantum efficiency (QE), being the amount of electrons emitted to the vacuum per incident photon. Some other materials have a negative electron affinity (NEA). In materials with a NEA the electron gains energy upon entering the vacuum, therefore the chance of being emitted to the vacuum is fairly high and the QE of the NEA cathodes is much higher than that of cathodes with a positive electron affinity. These NEA cathodes are known in that art.
As the QE is low, it can be improved by depositing an electron exit layer on the photo cathode layer, wherein the electron exit layer having a NEA. These depositions however have to be performed in ultra high vacuum and the bonding between III-V based cathode layer and the electron exit layer is based upon Van der Waals forces and therefore very weak.
Some (namely III-V based) cathodes with NEA properties and comprising an electron exit layer have a high QE, typically around 40%. The drawback of the use of an electron exit layer however is that, because of the weak bond, the electron exit layer has to be protected from chemical attacks of gasses emitted by the microchannel plate positioned inside the vacuum chamber of the vacuum tube and the phosphor screen applied on most anodes.
An other phenomenon to which the electron exit layer has to be protected is so-called ion feedback. This occurs when (negatively charged) electrons that have acquired sufficient kinetic energy in the accelerating electric field strike and ionise atoms or molecules still present in the vacuum or adsorbed at the surfaces stricken by the electrons.
Once the neutral gas atom or molecule has been positively charged by the electron impact that knocked an electron from the outer region of the atom's electron cloud, the ions are subjected to the same electric field but, due to their positive charge, will move in the opposite direction, acquiring kinetic energy and striking surfaces at the entrance side of the device.
These ion feedback impacts are quite often very noticeable and on most instances disturb or reduce the signal outputted by the device by so-called after-pulses or ion spots in the image of the device. In many of the prior art devices, special care is given to the design, the construction or the limitation in operating pressure range or operating voltages to avoid or reduce the effects of ion feedback.
As a common solution in the art, in particular in image intensifier tube devices having component surfaces made from or contain vulnerable mono-atomic negative electron affinity layers, like for example GaAs with a Cs-based surface layer, a so-called ion barrier membrane is disposed in the vacuum chamber in order to shield off those component surfaces from the stray ions. Such membrane will prevent that stray ions will permanently damage and reduce the cathode's emissive QE.
Using an ion barrier membrane however has an essential drawback. It not only blocks the feedback of stray ions, but it also considerably reduces the amount of primary electrons that can be considered to carry the signal or image information in the device towards the anode, resulting in a significantly lower emissive QE.